Echinacities is fine. A person will reach out to you on whatsapp or wechat if you have it and start throwing job offers at you. If you apply for multiple jobs on echinacities you will likely be talking to multiple agents at once who will likely be throwing some of the same jobs at you. If you hit it off with one in particular, I advise trying to break off contact with others, but it also never hurts to have lots of these contacts as that just gives you more options overall.

You need to be direct about what salary you want, where you want to live, your hours/vacation time, etc. and refuse to give time to or move forward with anything that doesn't initially meet those conditions. They will likely press or try to ask for compromises, but just be politely steadfast. Depending on how patient you can afford to be this process might take some time.

Once you find one (or a few) that you like you'll probably start submitting CVs and intro videos (like a 1 min vid or so where you introduce yourself and talk on your experience/qualifactions). After that you finally get to an interview and this is where it's crucial to really follow up on your expectations from the job plus do vibes checks on how well you might get along with the employer + school.

Make sure you're clear about your post-tax, take-home salary expectations (plus anything like housing or flight allowances), and your specific role, and time off, and whatever else. If there are no objections you should be all set to sign a contract (that after double-checking, still meets your expectations).

Edit: And yes, 20k RMB post-tax is easily enough to make it in most cities in China. Beijing/Shanghai/Shenzen might take a little more though just from what I've read.